In recent years, people have become increasingly aware of the importance of environment protection and energy conservation. Therefore, it is desirable that visualized information regarding CO2 emissions and electricity use be collected where appropriate and an optimum amount of energy be supplied. To realize such a requirement, an understanding of various situations on a site, such as electricity use and greenhouse gas emissions, is to be timely and accurately gained.
Accordingly, a sensor network that acquires data from widely distributed sensors that sense, for example, electricity use and greenhouse gas emissions has attracted attention. In particular, an ad hoc wireless network in which the sensors have a wireless communication capability and autonomously set up a network of a plurality of wireless stations has attracted attention.
The wireless stations in the ad hoc network may communicate with one another without using a base station and an access point. Accordingly, the ad hoc network does not depend on a particular network infrastructure. Thus, a new wireless station may be added to the ad hoc network without registration of the new connected wireless station and, therefore, a new wireless station may be easily added.
In this way, by employing an ad hoc network in order to realize the above-described service, an observation point may be added as appropriate and, therefore, an understanding of situations throughout a wide area may be gained.
However, if an ad hoc network is employed, the data transfer efficiency may be disadvantageously decreased among the wireless stations in the ad hoc network.
For example, according to the transmission time restriction for Telemeter and Telecontrol (including accompanied data) using a radio frequency in the range from 426.025 MHz to 426.1375 MHz defined in the ARIB STD-T67 standard, the maximum transmission period is 5 seconds and the minimum quiescence period is 2 seconds. If this restriction is imposed on a wireless station, the wireless station stops transmitting a packet within 5 seconds after it has transmitted the packet. In addition, the wireless station does not start transmitting any packet until 2 seconds have elapsed from the end of transmission.
Such a restriction is defined to give all of the wireless stations that use the above-described frequency range an opportunity to transmit a packet. In addition, for another unlicensed frequency range that is used by convenience radio stations, a transmission time restriction is defined. For example, each of ARIB STD-30, ARIB STD-T67, ARIB STD-T93, and ARIB STD-96 defines its own transmission time restriction.
That is, under such a transmission time restriction, if the maximum transmission period has elapsed since the wireless station started transmitting a packet, the wireless station is forced to stop transmitting any packet for the minimum quiescence period. Accordingly, the data transfer efficiency, that is, the number of transmitted frames per unit time is significantly decreased.
To address such an issue, a technology for increasing the data transfer efficiency by using a transmitter wireless station that multiplexes a plurality of communication data items destined for a plurality of receiver wireless stations has been developed (refer to, for example, Japanese Laid-open Patent Publication No. 2009-141507).